Precision Methods Used in Constructing Electric Wave 

 Filters for Carrier Systems 



By G. R. HARRIS 



Electric wave filters are used extensively in carrier telephone and 

 telegraph systems. In order that such systems may be operated efficiently 

 and economically, the requirements placed on the filters they employ are 

 occasionally so severe in nature that new methods in design or construction 

 must be developed to make the commercial production of the filters possible. 

 The band filters for the Type "C" Carrier Telephone System are cases in 

 point. This paper sets forth the requirements which were met in the design 

 of the filters for this system, and describes a new manufacturing adjustment 

 made necessary by these requirements. This feature consists essentially of 

 an inductance continuously variable over a small range above and below its 

 nominal value; the adjustment is not used to set the coil inductance at its 

 specified value, but to locate correctly the series or parallel resonance of the 

 mesh of which the coil is a part. The bridge and associated apparatus de- 

 veloped to facilitate this adjustment are also described. 



ONE of the most important fields of usefulness of the electric wave 

 filter in the Bell System is found in carrier current telephone and 

 telegraph systems. A carrier telephone system transmits several 

 messages over the same line by employing several carrier currents of 

 frequencies higher than those in the ordinary voice band, and modu- 

 lating these carriers with the messages to be transmitted. The 

 messages are then transmitted over the line as side bands of their 

 respective carriers, and are demodulated to their original voice fre- 

 quencies at the receiving end. 



Usually only one side band of a carrier is transmitted in order to 

 reduce the frequency space required for each channel. Each message 

 as it appears on the line occupies its own portion of the frequency 

 spectrum, distinct from that occupied by any other message, and is 

 transmitted without interference from other messages; but in order 

 that the operations of modulation and demodulation may be carried on 

 without interference and in order that unwanted side bands may be 

 suppressed, it is necessary that each modulator and demodulator be 

 equipped with some apparatus which will pass all the side band 

 frequencies making up one message and reject all others. Electric 

 wave filters are the instruments used for this purpose, since they 

 possess the property of passing currents of certain chosen frequencies 

 with very small loss, and of offering high attenuation to other chosen 

 frequencies. 



The theory of electric wave filters as used in carrier systems has 

 been discussed in previous articles in this Joirnial, notably "Physical 



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